The U.S. Pat. No. 3 377 176 discloses the production of a ceramic formed body made of Al.sub.2 O.sub.3 with a minute admixture of foreign oxides, whereby alum earth, the basic material, is pressed, said alumia having a grain size of &lt;1 .mu.m.
The U.S. Pat. No. 4 786 292 discloses the production of a sintered abrasive grain, said grain being derived from a mixture of alumina and highly dispersable .alpha.-Al-oxide monohydrate, whereby admixtures of oxides of various metals up to 1 weight percent are stated therein.
The European Patent Application EP-A 0 304 616 refers to surface treated abrasive grain based on alumina. The treatment of the surface may take place, amongst others, by means of highly disperse Al.sub.2 O.sub.3. The sole object of such treatment is, however, the achievement of high surface conductivity by applying hygroscopic and hydrophile layers. This publication does not disclose any items taking influence on the hardness.
The European Patent Application EP-A 0 263 810 refers to the production of a microcrystalline abrasive grain based on .alpha.-Al.sub.2 O.sub.3. The basic material used this case is highly dispersable .alpha.-Al-oxide monohydrate.
The European Patent Application EP-A 0 293 163 describes the production of an abrasive grain from an Al-oxide-sol, said grain, after having been calcined, being subjected to impregnation by means of a saline solution for improving its properties. The salts used in said saline solution are metal compounds which, during the subsequent heating, transform into metal oxides and react with the matrix.
The U.S. Pat. No. 4 007 020 discloses a method in order to bring about higher hardness in a porous, ceramic body. This takes place mainly by impregnating the porous structure with a solution of an anorganic chromium compound, said compound transforming into an oxide during heating. The impregnation is repeated several times, whereby, as a last step, said impregnation may be performed with phosphoric acid.
All above mentioned ceramic bodies, when used as abrasive grains in an abrasive disc, show high reactivity with the glass-like, ceramic binding material which is used in the production of discs. The surface of the grains reacts with the binder during the firing and said surface is "softened". Thus a decrease in the hardness on the surface of the grains occurs and, consequently, a decrease in the abrasive performance.
Disclosure of Invention
It is the object of the present invention to create an abrasive grain, the properties of which are not essentially impaired when said grain is bound by means of a ceramic binder.
A further object of the invention is to prevent a decrease in hardness on the surface of said grain and thus improved abrasive properties.
These tasks are solved in that the body, said body being bound into a structure by means of a glass-like, ceramic binder, shows a decrease in hardness from its center to its edge of less than 1800 N/mm.sup.2, preferably less than 1000 N/mm.sup.2, and in particular less than 500 N/mm.sup.2. Specifically, the decrease in hardness from the center of the grain to its edge was determined as being particularly adverse for the abrasive properties. The ceramic bodies according to the invention are capable of achieving extremely high abrasive results. Grain sizes between 0.05 and 1.5 mm and, in particular, between 0.2 and 1 mm have proved to be especially preferable.
The invention further refers to the use of a ceramic, sintered body for the production of abrasive discs and similar items.
In accordance with the invention, the grains are bound by a ceramic, glass-like binder. Abrasive discs produced in such a manner achieve the highest abrasive results.
A further aspect of the invention is a method for producing a ceramic body, said body being based on .alpha.-Al.sub.2 O.sub.3, by means of sintering. This method is characterized in that the presintered body is deaerated in a vacuum and said body is subsequently treated with an aqueous suspension, whereby the grain sizes of the solid matter in said suspension are less than 0.5 .mu.m, preferably less than 0.05 .mu.m, and even more preferably less than 0.02 .mu.m, whereinafter excessive suspension is taken away and the bodies treated in such a manner are dried and sintered again.
Quite surprisingly it was noticed that by treating near densely sintered bodies with a suspension of highly dispersable .alpha.-Al-oxide monohydrate, the decrease in the hardness on the grain surface, which was to be expected after the burning with the binder, was considerably reduced.
The known sintered, ceramic formed bodies comprise a more or less considerable residual porosity. When such grains are used in ceramic abrasive discs, said residual porosity has a negative effect on the abrasive properties. The glass binder comprises oxides with a low melting point and said oxides, during burning of the discs, enter into a liquid phase and penetrate the pores of the abrasive grain. This leads to a considerable chemical attack against the aluminum oxide located in the outer layers and along the grain boundaries. The consequence thereof is a strong decrease in the hardness of the grain surfaces which engage in the abrasive process.
The infiltration of the pores by the suspension, said suspension hardening in the pores during drying and sintering, brings about the effect that the liquid phase of the ceramic binder is no longer in the position to penetrate the structure of the grain and "soften" it.
In principle the infiltration may be performed in any sintered body still having open pores.
Any suspension is suitable the viscosity of which is low enough to penetrate pores whose sizes are between 0.5 and 1 .mu.m and where said suspension has solid matter consisting of .alpha.-Al oxide monohydrate and/or Al-oxide and/or Al-hydroxide. The grain size of the suspended particles should be less than 0.05 .mu.m in order to guarantee complete filling of the pores. The addition of acid to the suspension has proved to be advantageous. The content of solid matter in the suspension lies between 5-30 weight percent, preferably between 10 and 25 weight percent.
The infiltration itself can be performed by immersing the grains into the suspension and, subsequently, by filtering through a wide-meshed screen. During this process the grains are preferably deaerated, so that the pores can be well filled with suspension.
After getting rid of excessive suspension, the grains are dried. The increase in weight, depending on the porosity and grain size, may reach up to 15 weight percent. The dried product may also be carefully comminuted, in order to separate grains which are stuck together due to suspension remnants. Subsequently the final sintering of the filtered and dried grains takes place at temperatures between 1100.degree. C. and 1400.degree. C.
Like abrasive grains, said surface treatment may also be applied to sintered formed parts, e.g. cutting discs or expendable parts for improving the surface.